Coenzymes, prosthetic groups and certain inhibitors and activators (modifiers) of enzymatic reactions have frequently been observed to exert effects which are more profound than can be attributed solely to their participation in the catalytic reaction. It has been proposed that they effect conformational changes by binding either to an active site or to an "allosteric" site. The broad objective of this study is to understand in chemical terms the effect of coenzymes, or modifiers, on the course of enzymatic reactions. In order to accomplish this goal, it is necessary first to examine the nature of the functional groups at the active site of one or more particular enzymes and their role in the catalytic mechanisms. Any modification in the reactivity or interrelationshis between these groups which is conferred on the enzyme by the coenzymes or modifiers may then be investigated meaningfully. It is proposed to examine and compare at least two enzymes from this viewpoint: the TPN and DPN specific isocitrate dehydrogenases from mammalian heart muscle. The TPN-dependent enzyme is not known to be subject to control by modifiers, although some evidence suggests that its conformation is influenced by TPNH; whereas the activity of the DPN enzyme is modified by coenzymes and nucleotides. Both enzymes catalyze two sequential reactions: a pyridine nucleotide-dependent dehydrogenation followed by the decarboxylation of a B-keto acid. A detailed investigation will be conducted for each enzyme of the nature and reactivity of the functional groups which participate in these reactions in order to elucidate the distinction between an allosteric and non-regulatory enzyme.